The erythropoietic mouse spleen has been used as a model system to develop techniques (isolation and characterization of histone and nonhistone proteins from erythroid cell chromatin, assay from nuclear RNA polymerase) which will permit direct investigation of the efects of erythropoietin on the following events associated with the activation of transcription in splenic erythroid tissue: 1) synthesis o chromosomal proteins, 2) modification (acetylation, phosphorylation) of chromosomal proteins, 3) change in nucler RNA polymerase activity. In addition, the ability of antibodies directed against DNA-nonhistone protein complexes from mouse splenic erythroid tissue to inhibit the effects of erythropoietin and erythrpoietic activity in the mouse spleen will be investigated.